A polymer electrolyte fuel cell is constructed by sandwiching a solid polymer electrolyte membrane between an anode electrode and a cathode electrode to form a single cell, and by stacking the single cells up via electrodes called as a separator (or bipolar plate).
Materials for the separator for the fuel cell are required to have properties, including a low level of contact resistance (which corresponds to a voltage drop caused due to an interface phenomenon between the electrode and the surface of the separator), and maintenance of the low level of contact resistance for a long time during use as the separator. In this point, the use of metal material, such as an aluminum alloy, a stainless steel, a nickel alloy, or a titanium alloy, has hitherto been studied from the viewpoint of the above properties together with workability and strength.
Since the inside of the fuel cell is positioned in an acid corrosion environment, the material used for the separator is required to have enough acid resistance to maintain the low contact resistance for a long time even in the acid environment.
Metal material, such as a stainless steel or titanium alloy, exhibits good corrosion resistance by having a passivation film formed on its surface. Thus, such metal has been studied to be used as a material for separators for fuel cells. The passivation film, however, has a large contact resistance (electric resistance). When the metal, such as the stainless steel or titanium alloy, is used in the fuel cell separator as it is, the passivation film formed on the surface of the metal under the acid environment tends to drastically degrade electrical conductivity of the cell.
Thus, some techniques are proposed for suppressing an increase in contact resistance to maintain the electrical conductivity.
For example, Patent Document 1 has proposed a separator including a substrate made of a stainless steel, titanium, or the like, and an Au plating film attached to the surface of the substrate in a thickness of 10 to 60 nm.
Patent Document 2 has proposed a separator including an acid-resistant metal film formed using Ta or the like on the surface of a substrate made of stainless steel, and a noble metal film formed thereon.
Further, Patent Document 3 has proposed a metal separator including an oxide film included in a metal substrate itself, and a conductive thin film made of a metal, noble metal, or carbon as an outermost layer. Another metal separator has been proposed in which an intermediate layer made of Ta, Zr, Nb, or the like is formed between the oxide film and the conductive thin film so as to improve adhesion (adhesive strength).
[Patent Document 1]
Japanese Unexamined Patent Publication No. Hei 10 (1998)-228914.
[Patent Document 2]
Japanese Unexamined Patent Publication No. 2001-93538.
[Patent Document 3]
Japanese Unexamined Patent Publication No. 2004-185998.